Photo: Brent Stirton / Staff (Getty)
Rivers and streams cover much more of the planet than geologists previously estimated, according to a new study published in Science. In total, this new estimate shows that, excluding land with glaciers, Earth is covered by just under 300,000 square miles (773,000 square kilometers) of rivers and streams. That's as much as 44 per cent higher than previous counts.
The finding has implications for the study of climate change, because rivers exchange greenhouse gasses with the atmosphere, especially when humans pollute their waters.
"It was assumed until about 2006 that rivers and lakes were just a pipe transporting carbon to the ocean," John Downing, a limnologist and biogeochemist at the University of Minnesota Duluth, told Gizmodo. "But the rivers are leaking gasses into the atmosphere."
As pollutants like fertilisers and sewage seep into water supplies, gasses including methane, carbon dioxide, and nitrous oxide bubble out and drift up into the atmosphere. More river coverage could mean we're sending even more of those greenhouse gasses into the air than current calculations account for.
"Here's another reason not to spoil water," said Downing, who was not involved in the new study. "If you pollute it, you spoil fishing and swimming, but you also spoil the atmosphere."
To map out the planet's rivers and streams, University of North Carolina hydrologists George Allen and Tamlin Pavelsky analysed thousands of images from a NASA Landsat satellite. Using software developed by Pavelsky, the duo ended up with 58 million river measurements they then used to calculate the total coverage of rivers and streams on Earth.
And they didn't just blindly trust their software. To make sure the program was reliable, the researchers employed what Allen called "a small army of undergraduates" to vigilantly monitor whether the program was measuring rivers, not roads, and if it was avoiding other mistakes along the way. "They were so enthusiastic," Pavelsky said. "They did such a great job."
The researchers estimated river shapes by measuring their widths, as shown here.Graphic: George Allen, Tamlin Pavelsky ("Global extent of rivers and streams," by G.H. Allen; T.M. Pavelsky at University of North Carolina, Chapel Hill in Chapel Hill, NC; G.H. Allen at Texas A&M University in College Station, TX)
Aside from their Texas-sized estimate, the researchers also found that rivers were both narrower and more sparse in human-developed areas, which surprised them. This could be because people are siphoning water for agriculture, draining swamps, and leveeing rivers, but more research will have to be done to say for sure.
Other scientists have made estimates of Earth's river and stream coverage in the past. Downing, for instance, used the distinct way rivers branch and how much water they move to estimate their global coverage. Others have used elevation and water runoff data to do it. But none have shown river morphology — a river's shape and direction — like this satellite estimate does, the researchers said.
"If you look around the world, rivers look different from place to place," Allen, who recently began a professorship at Texas A&M University, told Gizmodo. "They might be braided, or sinuous, or meandering. And for the most part, current technology doesn't take into consideration the actual morphology of rivers. This data set is the first of its kind to do this at a global scale on high resolution."
Downing was impressed by the number of data points Allen and Pavelsky considered, and found their results exciting — especially because the duo's estimate was within 15 per cent of his own.
"I think it's absolutely terrific," Downing said. "They put in a massive amount of effort and confirmed the numbers we already had that were created from basic physical principles."
Having a map of Earth's river morphology may prove extremely useful when predicting floods, Downing added, or for studying the way rivers behave in the future as the Earth warms. Plus, now that this fine-tuned, river-measuring technique has some reliable results, he said it could be useful to see how river flow rates change from season to season — another good data set to have when working to keep communities safe from floods.
"Rivers are so wonderful, but they can also be dangerous," Downing said. "Water goes where it wants to go — so it's good to know where it goes and how much area it's covering."